Fruit sugar preferences of house finches

In a series of choice tests, we determined the relative preferences of House Finches (Carpodacus mexicanus) for equicaloric aqueous solutions of hexoses (1:1 mixture of fructose and glucose) and sucrose. At 2% (m/v), birds consumed each sugar solution equally and in amounts similar to plain water. Consumption of hexose but not sucrose increased at 4% sugar concentration. At 6% and 10%, finches displayed consistent, strong preferences for the hexoses over sucrose. In other passerine species, strong hexose preference has been linked to the absence of sucrase, the enzyme needed for digestion of sucrose. Fecal sugar readings from the House Finches, however, indicated approximately equal assimilation of hexose and sucrose, so the hexose preference apparently is not due to sucrase deficiency. Rather, energetics may determine the finches' sugar preferences: hexoses are rapidly processed because the 6-carbon sugars are readily assimilable whereas sucrose must first be hydrolyzed.